Belt-tension control device



Sept. 8, 1942.. i f E. K. HANSEN 2,295,401

BELT-TENSION CONTROL DEVICE Sept. 8, 1942. E. K. HANSEN 2,295,401

` BELT-TENSION CONTROL DEVICE Filed Feb. 20, 1940 2 Sheets-Sheet 2 INVENTOR .7n7ef Afb/005er?.

BYMM

ATTORNEY Patented Sept. 8, 1942 BELT-TENSION CONTROL DEVICE Elmer K. Hansen, Edgewood, Pa., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application February 20, 1940, Serial No. 319,871

11 Claims.

My invention generally relates to the tensioning of loaded belt conveyors, and more particularly relates to the tensioning of endless mesh-belt conveyors for ycarrying charges through a high temperature furnace for heat-treatment.

Conveyors used in heat-treatment lfurnaces are customarily made of commercially-available heat-resistant metals or alloys, a nickel-chromium alloy being mostly employed, especially Where the furnace is to be permeated by a controlled atmosphere for enveloping the charge. However, at the higher temperatures, which might range up to 2000o F., and even higher, such metals or alloys have only a small fraction of the strength which they possess at ordinary room temperatures, and are very sensitive to continually-applied forces, these forces being detrimental to the life of the belt, particularly if the belt is subjected to repeated heatings and coolings, as is usually the case with endless-belt conveyors for heat-treatment furnaces.

Under conventional methods of operating a lconveyor belt for a heat-treating furnace, a constant tension is maintained on the belt whose value is computed or adjusted so that the heaviest load anticipated on the belt will be moved. No consideration has heretofore been given to the effect of this constant tension on the belt'S-life with loads less than that of the heaviest anticipated. Under such conditions the expected life of a meshbelt is approximately 3,000 hours of operation. This means that a user will have to anticipate replacement of this costly item at fairly frequent intervals.

It is, accordingly, :a primary purpose of my invention to increase the life of mesh-belt conveyors subject to varying temperature conditions, including high temperatures appreciably decreasing the strength of the conveyors; and this I accomplish by controlling the tension on thebelt in accordance with its loading.

Consequently, it is a more specific object of my invention to provide a tensioning device for a belt conveyor of a heat-treating furnace, which will apply a tension to the belt generally in accordance with the variations of the loading of the belt, and, preferably, by decreasing the tension when the load decreases, and increasing the tension when the load increases.

Another object of my invention is to provide a tensioning device to be applied to an endless-belt conveyor for a heat-treatment furnace, which will adjust the pull on the belt in accordance with the maximum permissible stress at the point of highest temperature in the furnace whereby the tension thereat may be decreased when the load decreases so that the portion of the belt at this point of high temperature is subjected to lower stresses and strains with loads less than the permissible maximum. Thus, the pull on the belt, which is a lprime factor in determining its life, is controlled so as to keep destructive effects on the belt to a minimum.

In accordance with my invention, the endlessbelt is wrapped around a return tension drum or idler pulley which serves also to tension the belt just sufliciently to cause motion of the belt when a driving force is applied. To permit a force to act upon the idler pulley, and therefore, tension the belt, the pulley is rotatably mounted on a carriage-slidably mounted in a relatively fixed frame, and an automatic tensioning device has one end secured to the [carriage and the other end to the frame. This automatic tension device exerts varying forces on the carriage, depending on the position of the .carriage with respect to the frame, and therefore, responds to the load onthe belt. An adjusting device in the form of a screw is prorvlded for the frame so that stretching of the belt, in the course of time, can be compensated for by simply turning the screw. Consequently, the function of the tensioning means is confined purely to tensioning the belt under load.

The aforesaid objects and features of my invention as well as other objectives and features, will :be apparent from the following description, taken in connection with the accompanying drawings, of a preferred applic-ation thereof.

In these drawings:

Figure 1 is a schematic elevational view, partly in section, on a decreased scale, of a heat-treating furnace including an endless-belt conveyor for carrying the charge, and having my automatic tensioning device applied to the return pulley of the belt. This view shows the driving means for the belt at Aone end and the return pulley at the other end mounted on a slidable carriage riding in an adjustable xed frame;

Fig. 2 is a plan view of the carriage which slidably supports the return pulley;

Fig. 3 is an end View vof the carriage-supporting frame to more particularly show the manner in which it is supported upon a bed plate;

Fig. 4 is a diagram representing the balance of the forces and the relative position of the fonce members of the tensioning device when a belt has a relatively heavy loading;

Fig. 5 is a View similar to Fig. 4 but showing the distribution of forces and the force members when the loading of the belt is relatively lighter;

furnaces operating athigh temperatures, of

which a brazing furnace is a more specific example. Consequently, in Fig. 1, I schematically show a heat-treatment furnace 2 "comprlsin'gfa well-insulated heating chamber 4 havinga door; f

controlled entrance opening 6 and an exit opening 8 Which also is an entrance opening toa cooling chamber I forming part of the heat-treatment furnace. The charge is moved through the fur-` nace by means of an endless-belt I2 Wrapped over a drive drum lli-at the front end of thefurnace and'return wrapped over a return tensioning drum'4 or idler pulley I6 at'the'backfendof the furnace.l K The heating chamber is heated by any suitable means, such as electrical resistor units, While the vcooling chamber may have a water'- jacket about it so that the rcharge may be quickly cooled while it traverses this chamber. An 'endless-belt passing throughthev furnace will have its top'run'or upper portion subjected to high temperatures while' passing through the heating chamber, which will be cooled considerably when passing through the cooling"chamberl of the fur-` nace, while the bottom run or lower portion ofthe belt will be approximately at' room temperature since it passesl below the'bottomfofithe'-mair structure of the furnace. It will be at o'nce perceived, thereforef that the maximum permissible' stress-on the belt will be determined` bythe por I tion which happens to vbe in the heating chamber and which reachesthe highest'temperaturei. y Y The driving drum I4 is preferably driven by a motor y20 through a suitable power trans mittingsYv` means 22 which includes speed adjusting means so that the rate of travel of the belt maybe suit-A ably adjusted'. The drum I4 is preferably rubber covered and a pinch hroller` 24 contacts the outside of the belt for pinching the belt'to the drive drum. This pinch roller is also preferably rubber covered, andan adjusting means 26frna'y be employed to control the force with which'the pinch roller bears against the belt;

The drive drum is preferably located somewhat outward from Ythe entrance to the furnace'to,

provide a length of belt on the top rurrorfflight uponwhich charges can be loaded prior to their` entry into the furnace, and to provide a lengthj of belt closer to the drivedrumY which is slaclv and enables the tensioning device to function.

An apron plate 2'I is disposed below the front por-d tion of the top flight of the belt, and serves ,to

support this part of the belt, and also to peel the belt from the driverdrum.'

The idler pulley includes a central shaft 28 journaled in suitable bearings and housings n30 at each end Yof the shaft, the ,bearing housings being secured to a carriage 32 having wheels 34 at each of its corners. The carriage rides in` a frame, vrepresented inV its entirety by the reference character 33, comprising a skeleton frame# Work having horizontal trackv Ychannels 38 inY which the wheels 34 ride so that the idler' pulley is slidably'mounted with respect to the 'frame' 3S.; Removable stop lugs 39 may be provided at the ends of the track channels as a safety measure to prevent the carriage from riding off its tracks.

At each lateral side of the frame and carriage, a tensioning device is provided, each comprising a sprocket chain 4B, one end of which is secured to a lug 42 secured to a cross member 44 of the carriage 32. Thechain 40 is wrapped around a rotatably mounted sprocket/wheel 46 journaled in a bearing 4'I secured by any suitable rigid supporting structure to a longitudinally-directed side channel 48 of the frame 36 so as to be relatively stationary With respect to it. The other end of the chain is securedby any suitable means to one end of'alink'49 whoseother end is pivotally securedto the 'frame36, the mounting being represented bythe reference character 50. The link 4Q is preferably rigid, and at the end to which the chain 40 ris secured aweight 52 is pivotally attached, as Yat 53, to exert a pull on the carriage 32 and consequently control the tension onthe belt I2.t .Preferablyka single extended Abar is. employedasV 'the 'weight' 52 'for both tensioning devicesfthe bar extending 'across'the frame and being supportedv at' each end' from the pivot point E3 at each side ofthe frame." A Y p The belt I prefer to employ is this embodiment comprises a plurality of flattened helically-wound heatresista-nt alloy wires. A fragment of'such `a belt Vis shown 'in' Fig. 6 land' comprises thev helices 54, y with adjacent helices meshed with each other, and rods fpas'singthrough their meshing portions with the endsrof the rods securely welded to the far ends of the helices." rThis type of mesh belt is Well known and,' by itself, forms no part of my invention except as it enters into a combination with the remaining'featuresthereof Inorder to take upl thestretching of the belt and to initially adjustu theten'sion thereon, the frame 33 is slidably mounted in guide angles S9 secured to each side of a base plate 62, in turn xedly secured to the floor or foundation for the furnace. Consequently, the frame" 36 `may be slid or adjusted in `a direction parallelv to the pri# marydirection of lmovementn of the belt. Any suitable means may be provided to hold the frame adjustablystationary with respect tothe base plate. For'examplethe frame maybe adjustably secured to the base plate by means'of a long screw 64 passingthrough athreaded sleeveii which is fixed to the base plate `62, the ends of the screw co'operatingpf course, with any suit'-r -able means I 'lxed 'to the frame so that the frame will slide on the base plate when the screw is turned. One end of the screw is providedpwith an aperture 1,2, Yor otherappropriate;means, for receiving vavt/renale or lever for turning l.the screw in the sleeve 66, and consequently'moving the frame.

In designing a furnace of the type described,

the loading and the rate of productionareder-v termined by the tension in the belt approximate-9 ly atthe point C in the heating chamber 4, the

place ,of highest temperature, and is limited tol approximately pounds per footof beltjvidth, at 2000 F. for an 80% nickel, 20% chromium alloy. Izhis means that theweight of the loadvcar-V riedbywthe'beltand the belt from` the Apoint C to' the point A, the charge-loadingpoint fof the belt, multiplied bya friction factor must-not exceed 85 pounds per foot ofI belt, vfiolth. AIf' the weight sucientlygleree tegens@ motQn 9i thebelt., the direction @Une 'arrow Aallor/.11.1orrthe drive drum, whenthe entirey beltA is'` carrying amaxi mum permissible load to be heat-treated, there will be just the required tension at the point C.

If, however, all of the load to be heat-treated were removed from the belt, point C would still be under this tension with the conventional constant-tensioning means of the prior art, and there would be considerable tension in the belt at point A. The same would also be true if the belt were carrying lighter loads. Thus, under such prior constructions, the pull on the belt at point C would be greater than that required for actually moving lighter loads through the furnace. Consequently, by far the greater part of the belt in the heating chamber would be subjected to more tension than required to move the lighter loads with the result that, ultimately, the belt would fail sooner than it would have if the tension on the belt in the heating chamber had been decreased while the belt carried such ligh+ er loads.

By means of my tensioning expedient the tension on the belt at point C is always kept just enough to move the loaded portion between the points A and C so that the stresses in the belt at its weakest portions in the heating chamber are at all times kept to a minimum, with the result that the life of the belt is increased in furnaces in which the load to heat-treated varies from time to time. Stated differently, the tensioning means of my invention is such that there is practically no tension on the belt before the point A under all operating conditions.

In the operation of my automatic tensioning means, the screw 64 is employed for moving the entire frame to compensate for belt expansion from cold to operating temperature, or with inherent stretching of the belt with continued use. The tension on the belt during use is maintained by means of the weight 52 acting upon the chains lill through different angles determined by the rigid links 49. This is represented in Figs. 4 and 5.

With reference to Fig. 4, assuming that the belt is to move its maximum load, the carriage will tend to travel toward the left, whereby the lugs d2 on the carriage will pull on the chains 50 to turn the wheels 46 and raise the weight 52. Assuming a 290 pound weight and a suitable length of links 49, the angles .between the links i9 and the chains 4D extending from the links to the wheels 48 approach a horizontal straight angle, and the pull on the chains by the vertical weight increases, thereby increasing the tension on the belt. As a representative gure, I have been able to get a 500 pound pull on the belt under the assumed conditions, a drive-disconnecting expedient being employed when the carriage is moved to the left on the frame 36 to a point where the pull on the belt would be greater.

In Fig. 5, I show the resulting force diagram obtained when the load to be moved by the belt is decreased so that the carriage 32 moves to the right. Under this condition, the chains 40 unwrap over the wheels 4'6 permittingthe weight to drop, and the angle between the vertical force of the weight and the chains increases so that the component of pull on the chains is considerably reduced, thereby lowering the tension on the belt, the pull in this case being 140 pounds in the speciiic example described. At intermediate positions of the carriage representing intermediate loads to be moved by the belt, the pull on the belt will correspondingly vary.

Movement of the carriage 32 away from the furnace carries with it, of course, the idler pulley I6, and this requires a change in the stretched length of the top llight of the belt. This is obtained from A,the slack provided between the pinch roller 24 and the point A, the slack varying with varying load.

In Fig. '1, I schematically show a modified form of tensioning means using tension springs at each side of the carriage. In this case the lugs 42, the wheels 46 and their mounting means, the links 49, and all other related appurtenances for the variable linkage mechanism of the prior embodiment, are dispensed with. In the modification shown in'Fig. 7, each bearing housing 30 has secured thereto one end of a tension spring 80, the other end of the spring being held in an eye of an eyebolt 82 passing through aligned holes 84 in an upright channel 86 of the' frame 36.

By means of nuts 88 and 90, the tension on the springs may be adjusted, and by placing the eyebolts in different holes 84, vertically-spaced in the channels 86, therangle of the springs with respect to the carriage may be varied. In this embodiment, movement of the carriage to the left with increasing load stretches the spring and therefore increases the tension on the belt, and, correspondingly, movement of the carriage to the right with decreasing load decreases the tension yon the belt.

I claim as my invention:

l. A heat-treatment furnace of the type described, comprising a substantially horizontal relatively'high-temperature heating chamber having a charge-entrance opening at Vone endand a charge-exit opening atthe other end, a driving drum at said one end, a return drum at said other end, a heat-resistant, metallic, endless conveyor wrapped about said drums, said conveyor having one run passing through said chamber for carrying a charge through said chamber, and a return run outside said chamber, driving means for said driving drum, and means cooperating with said return drum for tensioning said one run and automatically increasing and decreasing the tension in saidV one run responsive to increased and decreased load carried by said one run.

2. A heat-treatment furnace of the type de= scribed, comprising a substantially horizontal relatively high-temperature heating chamber having a charge-entrance opening at one end and a charge-exit opening at the other end, a driving drum rotatably mounted at said one end, driving means for rotating said driving drum, a heat-resistant, metallic, endless conveyor having one run in said chamber for carrying a charge through said chamber, said conveyor being wrapped over said driving drum to be moved thereby, a return drum at said other end, said conveyor being wrapped over said return drum, said one run of said conveyor passing through said chamber in movingl from said driving drum to said return drum, a frame, a slidable carriage means slidable with respect to said frame, said carriage means rotatably supporting said return drum, variableforce tensioning means mechanically connectingsaid carriage means and said frame for automatically variably tensioning said one run in accordance with the reaction of said one run on said return drum, and means for adjustably nxing the position of said frame with respect to said driving drum.

3. A heat-treatment furnace of the type described, comprising a substantially horizontal relatively high-temperature heating chamber having a charge-entrance opening at one end and acharge-exit opening-iat the-other endgav drivingdrum rotatably mounted at saidfne endgv driving meansfor rotating said driving druml a l heat-resistant, metallic, endless conveyor-l1afvi`r-ga run' in said chainb-er forl-carrying lacharge through Vsaid -'-ch`ainber, saidf conveyorLi being wrapped over/#saldidrivingldrur tobe moved therebyg-a vrf-iturn drunr-atisaidother end, saidconveyor'l bin'gfwrappedover said return drum, a

frame; a Aslidable#carriagemeansslidable with repeatedly heatedand co'oledf a rotatable driving drunfarotata-ble return drum; said conveyor being" wrapped over 'said driving drum and said return drumistationary" 'power mean-s lgeared to saiddriving-'drumfor rotating said 'driving drum, a frame, means for xingthe position' of 1said frame with respect to said*driving-drunr,` supportingmeans for said return'drurn for slidably mounting saidreturndrum onv said frame;- and variable-'force means for automatically* controlling-the tension 'inf said conveyor, said Variable force'meanscompr'i's'ing alinkage systeml secured to said frameand-to saidV supporting means and also comprising fa weight secured to said linkage systemfor providing forces in said linkage system varying with-fthe positionr-ofsaid supporting means on said frame.-

5. A' device of the type described, comprising a conveyor, a rotatable driving drum, a rotatable return drum, said conveyor being -wrapped overA said driving drum and said return-drum; stationary power means geared to said driving drum for rotating said driving drum, a frame-,supporty ingmeansfor said return-Y druinforl-slidably mounting Vsaid return drum-on said--f-rair1`e`,a slidable carriage means slidable withre'speet to-said frame,I said carriage means rotatablyspporting said return drum, a' linka'gesystem lmec'hzm'icallyV secured to said 'carriage'andtdSaid-fiame" for' tensioning' saidA conveyor;` and Y a lfo'rce-'pi'oducing mezms'actingon saidV linkage system`inaccord ancewith the"re'lativepositionsof 4said carriage means and frame `for `producing :variable forcereactionsl of said linkageA system on said'icarriagev means, whereby to variably tension the conveyor.

6. A heat-treatment furnace of the type described, having a Ysubstantially horizontal relatively' high-temperature heating chamber including a charge-entrance opening at one endY and a charge-exit opening4 at the oppositeend, a conveyor comprising*an`endless-beltf comprising a heat-resistant"metall',- the strength of which is appreciably reduced2 at. 'the operating temperature' ofvsaid chamber; said" belt having a loadcarrying portion passing through said' entrance opening, said chamberfan'dsaid" exit opening in succession' for carryigacharge through said chamber, and havinga--returnportion outside said chamber, drivihgnieans-for'said bel-t including a, driving drum for said belt, a movable idler pulley for said'belt, thesaidlcad-carrying portionof said-"beltpassing through said heating chamber in moving from-saiddriving' drum` to" said movable idle-pulley, and means cooperatingv witliisaid-mevabllidler lpulleyfor tensioningthe first saidportinloflsaidbeltvariably in response-M to variable-loads tolbefmove'd byv said belt.V

7. A devie eftheclassrdescribed' comprising:

a conveyor comprising Ja Vheat-resistant endless belt of a lmaterial having'materially lesstensilef' strength-when hot;fa drivingv drum and an idler pulley -for the belt heating-means,` adapted to heat variousloads-*adapted -to be carried by a load-carrying portion of said belt,A at least a part of said load-carrying-portion beingv subjected to said'heatingimeans" arid Hheated-thereby to temperatures' atLvvhichF-the-v strength of at least' a part of said loax'i-carrying'4 portion is appreciably reduced, said-load-'carrying portion of said belt being' in -thatf part of Vthe belt ywhich moves from" said driving-"drumto said'idler pulley; and a variable-force? `ten'sion-ingmeans vfor tensioning said load-carrying-portion of said belt and 'auto'-` ,matically increasing and decreasing the tension' in said load-carrying portion inorder to auto-` ma'ticallyl'causea-ten'sio'n insubstantially the hottest' portion-ofsaid-@belt which will notl be` signicantlyfm'oi-e than that-necessary to ,m'oveY the loaded 'f partici-'the load-carrying portion of tures at which-thestrength-of'ati least a part of said load-carryingportion'is appreciably reduced in comparison? tofthe-strength of other cooler portions of the belt, lsucliloadlcarrying portion of said belt'being in-thatpar-tofthe belt which is operable to mo'v'efrinlsaid'fdriving druln to said idler pulley; terisionir'ig` meansfor said conveyoi, associated--with-said lidler pulley, said'tensioning means `comprisingmeanslfor automaticallyf tensioning'lsaidI belt, by 4displacing the axis` of said-idler pulley`,fo"r produinga substantially slack'portioni mLsaidbelt-'in the part -of the belt `between saidff driving-drum and; said loadcarrying portion; an'd'supporting' meansA for su'p'- porting` a Apartfoffthe belt-#which extends; from saidn slack `'portion' through the said part of the` beltfthe strength-oiwhich iis reduced as aforesaid.

describedcompris'ing: a-relatively?high` temperature heating-:chamber includinga charge-entrance' at-oneV end and-acharge"exit'at a second end; 'ai' conveyor "comprising `a hea-t-resistant metallic endless'l'belt having# aportion passing through said charge-entrance, said heatingchamber and said` charge-exit'-in'lsuccession, having a loading portion before said charge-entrance', and having fa' return' fportior outside said heatingchamber;' said-heati'n'- chamber`-being operative to produceL temperatures-at which -thej V'strength of atA least"l a part of'saidbelt exposed VtoV heating is materially reduced driving-means' for said belt, comprisingadrivingdrum somewhat before said.loadingportionof said -beltyand a movable idler* pulley; andfmeanscooperatingwith said movable idler pulley for automatically tensioning said belt so that it has substantially no tension in it at a point between said driving drum and said loading portion.

10. A heat-treatment device comprising: a heat-resistant endless belt of a material having materially less tensile-strength when hot; a driving-drum and an idler pulley for the endless belt; the endless belt having an approximately horizontal load-carrying portion on top, and a returnportion underneath; means for applying a turning-torque to the driving-drum and tensioning said belt, the direction of rotation being such that the load-carrying portion of the endless belt moves in the direction toward the idler. pulley, and the load-carrying portion of the endless belt is not subjected to any tensile-stress other than that necessary to move the load thereon, heatingmeans applicable to the load carried by the loadcarrying portion of the endless belt, and subjecting said load-carrying portion to heat; and supporting means for supporting said load-carrying portion of said belt.

11. A heat-treatment furnace of the type described, having a substantially horizontal relatively high-temperature heating chamber including a charge-entrance opening at one end and a charge-exit opening at the opposite end, a cont veyor comprising an endless-belt having a portion passing through said entrance opening, said chamber, and said exit opening in succession for carrying a charge through said chamber, and having a return portion outside said chamber, driving means for said belt including a driving drum for said belt, an idler pulley for said belt, the first said portion of said belt passing through said heating chamber in moving from said driving drum to said idler pulley, and means cooperating with said pulley for tensioning the first said portion of said belt variably responsive to various loads to be moved by said belt, said belt comprising heat-resistant metal, serially-arranged, relatively small, helically-shaped Wires arranged transversely of the direction of travel of said belt, adjacent helices being meshed, and heat-resistant metal rods through the meshing portions of adjacent helices to permit iiexing of the belt.

ELNIER K. HANSEN. 

